Roland Ver. 4.5 Technické informace stáhnout pdf (Strana 61)

Chapter 5: Writing Recipes in Rules 51

system. For example, suppose you have a sub-directory subdir which has its own makeﬁle,

and you would like the containing directory’s makeﬁle to run make on the sub-directory.

You can do it by writing this:

subsystem:

cd subdir && $(MAKE)

or, equivalently, this (see Section 9.7 [Summary of Options], page 104):

subsystem:

$(MAKE) -C subdir

You can write recursive make commands just by copying this example, but there are

many things to know about how they work and why, and about how the sub-make relates to

the top-level make. You may also ﬁnd it useful to declare targets that invoke recursive make

commands as ‘.PHONY’ (for more discussion on when this is useful, see Section 4.5 [Phony

Targets], page 29).

For your convenience, when GNU make starts (after it has processed any -C options)

it sets the variable CURDIR to the pathname of the current working directory. This value

is never touched by make again: in particular note that if you include ﬁles from other

directories the value of CURDIR does not change. The value has the same precedence it

would have if it were set in the makeﬁle (by default, an environment variable CURDIR will

not override this value). Note that setting this variable has no impact on the operation of

make (it does not cause make to change its working directory, for example).

5.7.1 How the MAKE Variable Works

Recursive make commands should always use the variable MAKE, not the explicit command

name ‘make’, as shown here:

subsystem:

cd subdir && $(MAKE)

The value of this variable is the ﬁle name with which make was invoked. If this ﬁle name

was /bin/make, then the recipe executed is ‘cd subdir && /bin/make’. If you use a special

version of make to run the top-level makeﬁle, the same special version will be executed for

recursive invocations.

As a special feature, using the variable MAKE in the recipe of a rule alters the eﬀects of

the ‘-t’ (‘--touch’), ‘-n’ (‘--just-print’), or ‘-q’ (‘--question’) option. Using the MAKE

variable has the same eﬀect as using a ‘+’ character at the beginning of the recipe line.

See Section 9.3 [Instead of Executing the Recipes], page 101. This special feature is only

enabled if the MAKE variable appears directly in the recipe: it does not apply if the MAKE

variable is referenced through expansion of another variable. In the latter case you must

use the ‘+’ token to get these special eﬀects.

Consider the command ‘make -t’ in the above example. (The ‘-t’ option marks targets

as up to date without actually running any recipes; see Section 9.3 [Instead of Execution],

page 101.) Following the usual deﬁnition of ‘-t’, a ‘make -t’ command in the example

would create a ﬁle named subsystem and do nothing else. What you really want it to do is

run ‘cd subdir && make -t’; but that would require executing the recipe, and ‘-t’ says not

to execute recipes.

The special feature makes this do what you want: whenever a recipe line of a rule

contains the variable MAKE, the ﬂags ‘-t’, ‘-n’ and ‘-q’ do not apply to that line. Recipe

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